The invention relates to a method for measuring the deformation moduluses of a soil sample in situ under conditions that are close to those of a triaxial compression test, and it also relates to apparatus for implementing the method.
Traditionally, direct measurement of the deformation moduluses of soil is performed by a xe2x80x9ccircularly-symmetrical triaxial compression testxe2x80x9d method. For this purpose, a corer is used to take soil cores, and the use of a corer is not without incidence on the quality of the sample obtained. In spite of the sophistication of corers, it is found that reorganization occurs when the core penetrates into the casing because of the arching effect created by friction between the soil and the inside wall of the casing. The intermediate stages between taking the core and performing the test proper allow the initial stresses in the sample to relax. In order to test a sample that is remote from this suspected reorganized boundary region, the sample is generally cut down well away from the outside edges of the core. This precaution gives rise to a significant loss of material.
Laboratory testing, referred to as triaxial compression testing, is a homogeneous test that serves to identify the deformation moduluses of the soil that are required for deformation calculations, e.g. of the xe2x80x9cfinite elementxe2x80x9d type.
Triaxial compression testing consists in placing the test sample (which is generally cylindrical in shape having a section that is circular or square) in a cylindrical bag that is expandable. The bag is placed in an enclosure full of liquid whose pressure can be adjusted, and it is placed between two pistons which can exert forces on the end faces of the sample. Measurement devices serve to measure the displacements of the wall of the bag, the displacements of the pistons, and the pressures inside the enclosure and inside the soil. The deformation modulus is measured either by increasing the pressure inside the enclosure and the forces applied to the piston, in which case it is variation in the volume of the sample that is measured, or else by injecting a known volume of liquid into the enclosure and then measuring the resulting variation of pressure inside the sample.
U.S. Pat. Nos. 4,502,338 and 4,579,003 describe instruments for testing the samples under triaxial compression conditions.
Triaxial compression testing serves to measure the deformation moduluses directly. It also serves to monitor drainage conditions and to determine the anisotropy parameters of the soil under test.
Nevertheless it suffers from various defects. Firstly it is cumbersome to implement and consequently expensive since the idea is to study soil while it is still intact.
It also requires a great deal of time since it is necessary during the consolidation stage to recreate the initial stress field prior to performing the test. The result is also liable to be biased by various errors, due to slack in the contact surfaces and to errors of axial alignment in the mechanical stack constituted by the test machine.
In order to avoid some of those drawbacks, proposals have already been made to perform soil strength testing in situ, by means of tools that are expanded in a borehole.
FR 1 596 747 thus proposes a pressure-measuring boring sonde which comprises a sonde body provided on its outer perimeter with a flexible diaphragm and in which there is placed a hollow rod, and a cutting tool constituted by a sonde body and by a soil breakup member disposed in the hollow body and secured to the hollow rod.
That self-boring sonde can perform measurements as the borehole advances. However, the drawback of the measurements performed with that sonde is that the pressure-measuring test is based on the sonde expanding in a cylindrical cavity formed in soil of infinite dimensions. Unfortunately, under such conditions, the stress state generated in the soil is not homogeneous, and as a result deformation moduluses can be obtained only by applying empirical relationships which are difficult to develop.
Use of the pressure-measuring test has become a requirement when dimensioning foundations. Nevertheless, in the field of retaining earth and landslides, laboratory testing still remains essential.
The object of the invention is to provide a method and apparatus that make it possible, in situ, to establish a stress field that is uniform in a finite volume of soil under test, while avoiding any movement of the sample of soil for testing before the actual beginning of each test.
Another object is to propose a method and apparatus for in situ measurement of the moduluses of deformation of a soil sample under circularly-symmetrical triaxial compression testing conditions which can be used in particular in the field of retaining earth and landslides.
According to the invention, the method is characterized by the following steps:
driving a corer into the soil, the corer being fitted on its inside face with a flexible diaphragm;
removing the material that leaves the corer through its top orifice as it is being driven into the soil, so as to form a sample of soil for testing inside the corer;
holding the corer stationary in the soil in order to proceed with a measurement;
exerting pressure forces on the outside face of the diaphragm and on the top face of the sample so as to subject the sample to radial and axial stresses; and
measuring the deformations to which the sample is subject.
Preferably, during measurement, the sample is subjected to predetermined radial stresses, and the axial stresses are varied until the sample breaks.
In order to determine the anisotropy parameters of the soil, a corer is used having an inside section that is substantially square.
The apparatus of the invention for implementing the method comprises:
a corer fitted on its inside face with a flexible diaphragm;
means for driving said corer into the soil in order to form a sample inside said corer;
means for removing the material which leaves said corer through its top orifice while the corer is being driven into the soil;
means for holding said corer stationary while taking a measurement;
means for applying an axial force to the top face of the sample;
means for applying pressure to the outside face of the flexible diaphragm;
means for measuring the radial and axial stresses to which the sample is subjected; and
means for measuring the deformations to which the sample is subject.